Neurexins comprise a family of neuronal cell surface proteins present primarily in the brain. Neurexins are thoμght to serve as signaling receptors and may play a role in axon guidance and synaptogenesis. The cytoplasmic tail of neurexin interacts with the synaptic vesicle protein synaptotagmin and binds to CASK, a protein with homology to the PSD95 family of synaptic junction proteins.